Methods for preparing luminescent materials



- formed and then placed in a'fused quartz beaker;

2,956,028 Patented Oct. 11, 1960 ice 7 METHODS FOR PREPARING LUMINESC'ENT MATERIALS Joseph A. Davis, Marion, Ind., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed May 23, 1958, Ser. No. 737,216

Claims. (Cl. 252-3016) This invention relates to methods for preparing luminescent materials, and particularly but not necessarily exclusively, to improved methods for preparing silver-activated zinc and zinc-cadmium sulfide phosphors.

Luminescent materials, or phosphors, are generally prepared by firing at an elevated temperature a mixture of host crystal material, one or more of activators as salts thereof, and a flux. The flux comprises one or more compounds usually halides, and is generally believed to serve the following purposes:

(a) Aid recrystallization of the host crystal.

(b) Aid incorporation of the activator into the host crystal.

(c) Reduce firing temperature.

(d) Control particle size of the product.

(2) Provide and incorporate a charge compensator into the host crystal.

For certain applications, it is desirable to shorten the persistence of zinc and zinc-cadmium phosphors. Previously, this was accomplished by activating these host crystals with silver and nickel. This double activation however, has the efiect of reducing the luminescence light output.

An object of this invention is to provide improved methods for preparing luminescent materials.

Another object is to provide methods for preparing luminescent silver-activated zinc and zinc-cadmium sulfide phosphors with improved light output and shortened persistence.

In general, the objects of the invention herein are attained by the use of a combination of three particular compounds as the flux during the synthesis of the luminescent material. Specifically, it has been found that an improved product results from firing the host crystal material and an activator together with the combination of flux compounds:

Magnesium chloride 1.0 to 4.0, preferably 2.0, weight percent Ammonium chloride 0.5 to 4.0, preferably 2.0, weight percent A Trisodium phosphate 0.05 to 2.0, preferably 0.5 'weight compounds: two grams magnesium chloride (MgCl),

two grams ammonium chloride (NH Cl), and 0.5 gram trisodium phosphate (Na PO The flux compounds may be in the anhydrous or hydrated form. This mixture is slurried and allowed to dry at 120 C. overnight. The dried mixture is lightly ground to break any lumps The beaker is covered with a loose fitting lid and placed in a 7 cent of sulfur.

furnace at 980 C.i10 C. for a period of about minutes. The sample is allowed to cool in air after which the material at the surface of the fired batch is removed and discarded. The remaining fired batch is then washed free of soluble flux compounds with portions of demineralized water and dried overnight at 120 C. The dried phosphor is passed through 200 mesh screen and stored for subsequent use.

The phosphor of the example is used as the blueemitting component in P-4 type screens of kinescopes for black and white television. Such screens are formed by mixing the blue-emitting component with a yellow-emitting component and then settling the mixture through an aqueous medium upon the face plate of the kinescope. A common problem with P-4 screens is that the blue-emitting component has a lower light output and a longer persistence than the yellow-emitting component. The blue-emitting phosphor of the example has a light output 10 to 20 percent higher than conventional blue-emi-tti'ng silveractivated zinc sulfide phosphors. The phosphor of the example also has a persistence shorter by a factor of 2 to 3 than the persistence of conventional silver-activated zinc sulfide phosphors. The conventional silver-activated zinc sulfide phosphor used for a comparison is prepared by a process similar to that of the example except that the flux is two weight percent of a combination of sodium chloride and calcium chloride.

Host crystal.-The improvements herein are obtained with 'zinc sulfide host crystals. Cadmium may be substituted for up to about 10 mol percent of the zinc and selenium may be substituted for up to about 10 mol per- The' host crystal material is introduced into the batch as an ultrapurity luminescent-grade form of the compound itself. I

Activatorsx-Silver may be replaced with one or a combination of known activators. Some suitable activators are copper, manganese, and gold. Activators may be used in proportions between 0.001 to 0.1 weight percent. In the case of manganese, up to 5 weight percent may be used. T Activators are introduced as any convenient salt thereof. Silver may be introduced for example as silver nitrate, silver chloride, silver bromide,

. silver iodide, or, as in the example, as silver sulfide mixed following with or incorporated in the hosttcrystal material.

F lux.The flux in. this invention is limited to the following combinations of flux compounds: 1

Magnesium chloride 1.0 to 4.0, preferably 2.0, weight percent f Ammonium chloride 0.5 to 4.0, preferably 2.0, weight percent Trisodium phosphate 0.05 to 2.0, preferably 0.5, weight percent W The effect of varying the proportion of flux compounds within the foregoing ranges is as follows:

Increasing the concentration of either magnesium chloride or ammonium chloride above 2 weight percent will shorten persistence still further. However, it is accompanied by an increase in particle size and decrease in light output. Decreasing the concentration of magnesium chloride or ammonium chloride below 2.0 weight percent yields phosphors with longer persistence than that obtained employing the preferred concentration.

Increasing the concentration of trisodium phosphate produces both an increase in light output and a shift of color toward the green region of the spectrum. The maximum shift occurring at the 0.1 weight percent con centration. Little shift occurs above this concentration. The maximum light output is obtained with 0.5 weight percent. Trisodium phosphate does not materially affect the persistence.

Firing.--Firing is carried out in any manner commonly used in the phosphor art. It is preferred to use a temperature between 900 and 1100 C. for the host crystals herein. -It is also preferred to fire in an atmosphere that of silver, copper, gold and manganese together with the following combination of flux compounds:

Magnesium chloride 1 to 4 weight percent Ammonium chloride 0.5 to 4.0 weight percent is chemically inert to the batch being fired. In the ex- 5 T hos hate 9 05 to 2 Wei ht ercent ample, the batch is fired in a covered beaker. The batch n 1 p p g P could also be fired in a neutral atmosphere, such as nitro 9 Q Preparmg a luminescent mammal gen or argon- Increases in firing temperature produ e comprising firing 2111C sulfide and between 0.001 and 0.1 a longer persistence and a lower light output. Decreases PRP P an actwator Selected from the group in firing temperature produce the reverse eifect in the 10 conslstmfl of Saver P f and manganese product. gether with the following comb1nat1on of flux compounds:

The firing time depends upon the size of the batch Magnesium hl ide 1 to4weight percent and the temperature. The smaller the batch the shorter A niu chloride 0.5 to 4.0 weight percent the firing time. For batches of one pound or less, 45 T i diu phosphate 0.05 to 2weight percent minutes is sufiicient. For six pound batches, 150 minutes A h d f 1 may be required. The higher the firing temperature, the Z [malt o or preparing a ummescent m Show:r the firing tinm prising firing z1nc sulfide, 0.001 to 0.1 weight percent Tests.--To determine light output, the phosphor sample as a salt f wlth the followmg combmauons is settled through an aqueous silicate solution upon a O ux compoun 2" x 2" glass plate. The settled plate is dried and placed Magnesium chloride 1 to 4 weight percent in a demountable cathode ray apparatus. The coating is Ammonium chloride 0.5 to 4.0 weight percent then exclted with a cathode ray beam raster. The rela- Trisodium phosphate 0.5 to 2 weight percent 2 5 5 :2 33 ggl then compared mth the vanous 4. method for preparing a luminescent material To determine emissim c 1 th fr th comprising firing at a temperature between 900 and 1100 foregoing X plates g i i g g igg z C. zinc sulfide, 0.001 to 0.1 weight percent silver as a recording spectroradiometer. Emission intensity versus igg with the following combmanons of Comwavelength is plotted. p0

To determine persistence a sample of the dry phosphor MagneslPm chlol'lfle 1 to 4 Welght Percent powder is placed in a demountable cathode ray apparatus m q chlonde to Percent and excited with 8 kv. square wave cathode ray pulses. Tnsodlum Phosphate tozwelght Percent The intensity of emission is measured during excitation 5. A method for preparing a luminescent material and at 10 and 40 milliseconds (ms.) after removing comprising firing at a temperature at between 970 and the excitation. The recorded values are percent of the 990 C. for a period of between 45 to 150 minutes a emission intensity during excitation, mixture consisting essentially of zinc sulfide containing Dam-The table gives data for a representative group about -9 Welght Percent Silver. and the fOHOWlflg of phosphors prepared according to the invention comcombmauon of flux compounds: pared with previous phosphors. All of the phosphors of Magnesium hloride 2 weight percent the table are silver-activated zinc sulfide ZnSzAg (0.011) Ammonium chloride 2 weight percent prepared according to the example except as listed. Trisodium phosphate 0.5 weight percent Table ICI Color Percent Firing Coordinates Persistence Visual Type of Flux at- Etfi- Sample 'Ifiirne, Tergp, ciency Size in. z y 40 ms 10 ms.

0.103 5.0 15.0 120 980 91 51b. 0.100 5.0 11.0 120 980 96 51b. 0.100 1.7 4.8 95 980 107 51b. 0.105 2.1 0.3 980 105 11b. 0.109 1.0 4.1 45 980 113 100 g. 0.111 2.2 5.3 40 980 1015 100g. 0.110 as 6.6 980 96.7 100g. 0.112 2.7 5.8 00 240 101.4 100 g. 0.112 2.5 0.1 60 980 101.3 100 g. 0.084 3.0 5.5 40 980 75.3 100 g. 0.082 2.0 4.9 40 980 70 100g.

1=Stai1dard flux mixture: total 2 weight percent NaC1+CaO1 NH CL 2=2% MgClz-I2% NH4C1+0.5% NznPO low persistence flux mixture.

What is claimed is:

1. A method for preparing a luminescent material comprising firing a zinc sulfide wherein 0 to 10 mol percent of the zinc therein is substituted with cadmium and 0 to 10 mol percent of the sulfur therein is substituted with selenium, and between 0.001 and 0.1 weight percent of an activator selected from the group consisting References Cited in the file of this patent 

5. A METHOD FOR PREPARING A LUMINESCENT MATERIAL COMPRISING FIRING AT A TEMPERATURE AT BETWEEN 970 AND 990*C. FOR A PERIOD OF BETWEEN 45 TO 150 MINUTES A MIXTURE CONSISTING ESSENTIALLY OF ZINC SULFIDE CONTAINING ABOUT 0.011 WEIGHT PERCENT OF ZINC SULFIDE CONTAINING COMBINATION OF THE FLUX COMPOUNDS: MAGNESIUM CHLORIDE 2 WEIGHT PERCENT AMMONIUM CHLORIDE 2 WEIGHT PERCENT TRISODIUM PHOSPHATE 0.5 WEIGHT PERCENT 